The invention relates to a coil for producing magnetic fields having high or very high homogeneity.
In order to measure the properties of liquid media by nuclear magnetic resonance, use is made of a very homogeneous static magnetic field H.sub.o, which is generated either by an iron magnet, a permanent magnet, by air coils or by superconductive coils, in conjunction with a magnetic high-frequency field 2H.sub.1 cos .omega..sub.o t, where .omega..sub.o is the resonance frequency in the field H.sub.o of the nuclear spin under investigation (A. Abragam, "The Principles of Nuclear Magnetism", Clarendon Press, Oxford, 1961 and A. Losche "Kerninduktion", VEB Deutscher Verlag der Wissenschaften, Berlin, 1957). If H.sub.o is not sufficiently homogeneous, the effect of inhomogeneity can be partly compensated by using "nucler spin echoes" and the measurements can be performed as if the H.sub.o field was considerably more homogeneous (see the aforementioned citations). To this end, use is made of a sequence of high-frequency pulses, the lengths and intervals between which are adjusted in accordance with the particular measurement. This experiment is more successful in proportion to the homogeneity of the high-frequency amplitude H.sub.1 used. The problem therefore arises of making H.sub.o and H.sub.1 as homogeneous as possible throughout the volume of substance under investigation.
If, more particularly, the substance under investigation has a large volume of measurement (e.g. in the case of mass flow measurements by nuclear magnetic resonance), H.sub.1 is non-uniform in a normal cylindrical coil, which must be relatively short in the first case (coil length.apprxeq.diameter). This also applies to the coil proposed by R. H. Lyddane and A. E. Ruark, Rev. Sci. Instr. 10, 253, 1939), hereinafter called the LR coil. In most cases, the homogeneity is worse in proportion to the ratio of the diameter of the substance under investigation to the diameter of the coil. The same effect occurs in the case of H.sub.o, if this static magnetic field is generated by an air coil or a superconductive coil. A further difficulty in the case of the high-frequency (H.sub.1) transmitting coil is that if the volume is large, the high-frequency power has to be considerable, and this results in a considerable high-frequency voltage. This means that the turns in the coil must be well insulated from one another. The LR coil, which initially has only two turns, is fairly homogeneous, but has a small H.sub.1 field at its center. If the coil comprises two stacks of turns and the turns are close together, H.sub.1 becomes larger but the insulation problem becomes almost insoluble, since oscillating voltages of the order of 10,000 V occur.
It is known to solve the aforementioned problem by using a cylindrical coil, which is relatively inhomogeneous and has a considerable leakage field at its ends, i.e. the field is not sharply defined at the ends. The interior of the coil is accessible only from its ends. Insulation becomes difficult if the high-frequency voltages are considerable. The field extends axially inside the coil. However, transverse magnetic fields H.sub.o and H.sub.1 are required for measuring mass flow at a loop or discharge duct, using nuclear resonance. The cylindrical coil has the previously-mentioned disadvantages even when the field inside the coil is made more homogeneous by specially arranging the turns. Consequently, a cylindrical coil is not suitable for measuring mass throughput by nuclear resonance, either as a field coil or as a high-frequency transmitter coil, but can be used as a high-frequency receiving coil. Admittedly the LR coil has a transverse field and its interior is accessible from all directions, but is not sufficiently homogeneous as a high-frequency transmitter coil and field coil when the volumes are large. When the high-frequency field strength is considerable, an LR coil needs to have a number of closely adjacent turns, which are difficult to insulate from one another, as previously described.
An object of the invention is to provide a coil of the aforementioned kind which can be used to generate very homogeneous magnetic fields and, more particularly, provides a homogeneous magnetic field over a large volume of sample under observation.